Oxide cathode more particularly for a gas-filled discharge tube



H. LEMS OXIDE CATHODE MORE PARTICULARLY FOR A GAS-FILLED DISCHARGE TUBE Filed Sept. 7, 1934.

Patented Feb. 15, 193s PATENT OFFICE o xID'E cs'rnoni MORE PARTICULARLY FOR A GAS-FILLED DISCHARGE TUBE Hendrik Items, Eindhoven, Netherlands, assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application September 7, 1934, Serial No. 743,139 In Germany September 14, 1933 a 4 Claims.

The present invention relates to a novel cathode construction for discharge tubes, and more particularly to a cathode construction having a large electron-emissive area. The cathode structure of the invention is especially useful in gas-filled discharge'tubes, for instance, rectifier tubes, adapted to handle large currents.

To obtain a high efficiency cathode, its surface is activated by providing it with a highly electronemitting layer, usually with an oxide layer. In such cathodes, when adapted to handle large currents, a large amount of oxide is to be distributed over a large surface area and the oxide must firmly adhere to the surface of thebody carrying the oxide layer. If such cathodes are notproperly degasified, the occluded gases liberated during the operation of the tubesdeleteriously influence the proper operation of the tube and also reduce the life of the cathode.

'Various constructions have been already suggested to obtain in a comparatively compact form such cathodes of large electron-emitting surface area.

For example, it has been proposed for such purpose to form a cathode'of a system of concentric cylinders which are interconnected by radial walls and provided with a centrally disposed heating element, whereby the surface of the cylinders and of the radial walls was coated with an oxide layer. However, such and similar structures have the drawback that it is difficult to readily and completely degasify same.

For gas-filled rectifier tubes adapted to handle comparatively small currents, a construction has been suggested in the patent to Dobben et al. No. 1,718,123, which provides for a cathode structure comprising a core of refractory metal, for instance of tungsten, around which is wound a coiled nickel wire, the adjoining turns of the-nickel wire being in intimate contact with each other. The whole cathode body is provided with a coating of highly electron-emissivesubstance, for instance, a coating of alkaline earth metal, which firmly adheres to the nickel. Such a cathode can be readily deprived of all occluded gases by passing a high-intensity heating current through same.

The present invention provides for a compact cathode construction having a large area and highly electron-emitting surface, which at the same time has the advantage of easy degasifying obtainable with the construction of said prior patent.

According to the invention, a plurality of coile Wires are disposed within each other, preferably concentrically; the individual wires consisting preferably of a core wire of a refractory metal, for instance of tungsten, which is covered with a coiled nickel wire whose adjacent turns are in contact with each other.

During operation, the heating current passes through one, more or all of the coiled wires, so that the cathode structure is heated both directly and indirectly.

The coils of the assembly can be degasified individually, or simultaneously e. g. by sending a series current through all of them if the construction permits. After degasifying this results in a compact large-surface area cathode struc ture which gives excellent performance and long life.

'When used in a gas-filled discharge device, for instance in a rectifier tube, the gaseous filling may be either an inert gas, or a vapor or a mixture of the two.

My invention will be more clearly understood by reference to the accompanying drawing.

Figure 1 is a view of a coiled wire cathode of a construction very commonly used in present day practice.

Fig. 2 is a schematic view showing one embodiment of the invention.

Fig. 3 is a top view of a discharge tube having a cathode structure according to the invention;

Fig. 4 is a partly secticnized side view of the discharge tube shown in Fig. 3.

In Fig. 1 the core l consists of a refractory metal, for instance tungsten, a coiled wire 2, preferably of nickel, covering the core with closely wound turns and ensuring a firm adherence of the electron emitting layer to the cathode surface.

In Figure 2 the cathode coil l consists of a coiled tungsten wire surrounded by a nickel wire, the entire coil being provided with a surface coating of a high electron-emitting substance, for instance with barium oxide. The cathode coil is provided with supply leads 2 and 3. Within the coil I is disposed a second coil l of a construction similar to that of coil l and having a smaller diameter, the two coils being preferably concentric. One end of the coil 4 is connected at 5 with the lead 2 of cathode coil l, whereas the other end of coil t is provided with a separate lead 6. The leads 2, 3 and 5 are sealed to the envelope of the a discharge tube in known manner.

In Figs. 3 and t a rectifier tube comprises a bulb l containing a cathode constituted by two concentrical coiled tungsten wires 2 and 3, which are covered with a layer of emitting material adhering to an intermediary layer of nickel wire wound with adjacent turns upon a tungsten core.

The coiled wires 2 and 3 are supported by a common metal rod 4, and two separate rods 4', 4" welded to sector shaped plates of special alloy 5, 5, 5" respectively, to which the lead wires 6, 6', 6"are welded in their turn. The plates 5, 5, 5" are hermetically sealed to the glass bulb of the rectifier tube and through glass to each other, for instance in the manner described in the U. S. Patent #1334509.

The rods 4, 4,'4 -arlLsurrounded by porcelain tubes 1, l, 1, respectively, which tu glass sleeves 8, 8', 8", fused to the glass covering the plates 6, 6, 6", so as to form an absolutely tight protective layer, insulating the metal parts of the leading in construction from each other and from the discharge space.

Supported fromgaorcelain tube I is a wire 9 ,,d- C9l1ryll1g a piece of a suitable getter material I0.

The wire is bent to a shape permitting the getter to be retained in the position shown throughout the pumping process, thus at a suflicient distance from the incandescent cathode 23.

After the pumping process the piece of getter material I0 is slipped over the upwardly bent part of wire 9 and caused to drop to the coiled part II in close proximity to the incandescent cathode 2--3.

Then the getter material I0 is evaporated by the heat generated by the cathode in order to clean up'the tube.

At the bottom of the cathode bulb a mercury container 23, filled with a small quantity of mercury, protrudes downwards from the cathode bulb proper and serves to maintain the mercury vapor pressure constant and at a value corresponding to a temperature slightly above that of the surroundings.

To the cathode bulb I are fused two arms I2, each of which contains an anode I3 which is screwed onto a rod I4, this rod being led through the wall of the anode-arm by the intermediary of a disc I5 in the manner set out above with relation to the cathode construction of this tube.

The anodes I3 are closely surrounded by metal gauze caps I 6, which confine the negative glow to the front surface of the anodes I3.

The grids I! are provided closely to the anodes l3 and consist of concentrical cylinders I8, I9 interconnected by radial partitions 20. This grid construction is provided with an outside connection at 2| At 22 an auxiliary anode is provided in order to improve the ignition conditions of the tube.

Referring to Fig. 2, the cathode coils I and 4 are degasified during the exhaust of the tube by their being heated individually to the proper temperature, whereby current is individually passed through coils I and 4.

In the operation of the tube either pair of supply wires 2-3 of 2-6 or both pairs may be used for the passage of the heating current through the respective coils. As a rule it sufiices to directly heat either the outer coil I or the inner coil 4, whereby the other coil is indirectly heated.

Normally the oxide layer provided on the coils is sufiicient to properly insulate them from each other. However, if desired, insulating rods, indicated in the drawing by 'I and 8, for instance of procelain, can be interposed between the coils.

It should be well understood that instead of using only two coils so placed within each other, the cathode structure can be formed of a larger number of coils disposed within each other in a similarnianner. Thereby each coil is preferably connectedwith one of its ends to a common lead through which the discharge current passes during operationand with its other end to individual leads and for degasifying, each coil is individually heated during the evacuation of the tube, whereas in operation the heating current is sent through an optional number of coils.

While I have described my invention orrhanw of specific embodiments and speclfic a'ppli'cation, sh to be limjtedfthereto, but desire the appended c t'p l e construed as broadly as permissible in view of the prior art.

What I claim is: V -l m 1. A discharge tube'coriiprising an envelope, electrodes within said envelope including a cathode structure comprising a plurality of cylindrically-shaped coils concentrically-disposed in a heat-retaining relationship, said coils being electrically interconnected and spaced apart to permit the discharge to readily reach both the internal and'external surfaces of each coil, a layer of a highly electron-emitting substance on the surface of said coils, and leads for supplying discharge current to all of said coils and for supplying heating current to only a portion of said coils, the remaining portion of said coils being indirectly heated from the directly-heated coils.

2. A discharge tube comprising an envelope, electrodes within said envelope including a cathode structure comprising a plurality of cylindrically-shaped coils disposed one within the other in heat-retaining relationship, said coils being spaced apart and having spaced turns to permit the discharge to readily reach all portions of the surfaces of all the coils, a layer of a highly electron-emitting substance on the surface of each coil, a lead for supplying cathode-heating current and discharge current to said cathode structure, said lead being connected to one end of each coil, and heating current leads connected to the free ends of a portion of said coils, the remaining portion of the coils being indirectly heated.

3. A discharge tube comprising an envelope, electrodes within said envelope including a cathode structure comprising a plurality of cylindrically-shaped electrically-interconnected coils disposed one within the other in heat-retaining relationship, said coils being spaced apart and having spaced turns to permit the discharge to readily reach all portions of the surface of each coil, a layer of a highly electron-emitting substance on the surface of said coils, and means including leads within said envelope to supply discharge current to all the coils and to directly heat only a portion of said coils during the operation of the tube, the remaining coils being indirectly heated.

4. A discharge tube comprising an envelope, an anode, and a cathode structure comprising two cylindrically-shaped coils disposed one within the other in heat-retaining relationship, said coils being spaced apart and having spaced turns to allow the discharge to readily reach the entire surface of both coils, a layer of a highly electronemitting substance upon said coils, a lead connected to one end of each of said coils, a heatingcurrent lead connected to the free end of one coil, and a degasifying-current lead connected to the free end of the second coil.

HENDRIK LEMS. 

